Euler Lab is a powerful suite of tools for numerical math and symbolic algebra (via Maxima).
Excellent graphics output - 2D, 3D, even photo realistic scenes - makes it particularly suitable for anyone who needs to create or present mathematical illustrations.
It's a hugely versatile system with vast scope for expansion, but fortunately you don't have to spend an age mastering every fine detail to see if the program is right for you.
Click File > Open Tutorials or Examples to explore the many sample files bundled with the project.
Select "03 - 3D Graphics", for instance, press Ctrl+End to jump to the end of the document, then Edit > Run Commands in Section. Not only will Euler Math Toolbox plot the function in 3D, it'll also animate it and display the results (press Esc to make it stop).
If you're still not sure, check out the Examples page for some more ideas on what the program can do.
A versatile Matlab-like suite which can also produce good-looking graphics and illustrations.